Elastomeric graft polymeres

ABSTRACT

Particulate elastomeric graft polymers which are suitable for improving the quality of macromolecular base materials B consist of 
     a) a grafting core comprising an elastomeric polymer P K , the particles having a mean particle diameter of from 1 to 150 μm, and 
     b) one or more shells, the outer shell S a  consisting of a polymer P a  which is compatible or partly compatible with the base material B.

The present invention relates to particulate graft polymers which aresuitable for improving the quality of macromolecular base materials Band consist of

a) a grafting core comprising an elastomeric polymer, the core having amean particle diameter of from 1 to 100 μm, and

b) one or more shells, the outer shell S_(a) consisting of a polymerP_(a) which is compatible or partly compatible with the base material B.

The present invention furthermore relates to the preparation of thesegraft polymers, their use for improving the quality of macromolecularbase material B and mixtures of these graft polymers and the basematerials and moldings produced therefrom.

It is generally known that the impact strength of nonresilient polymerscan be improved by adding elastomeric polymers. In such blends, thenonresilient base material, which is also referred to as the matrix,forms a continuum in which the elastomeric particles are incorporated inthe form of discrete particles.

Since the elastomeric polymer is often not compatible with the basepolymer and is therefore difficult to incorporate in the base polymer,the rubber is surrounded in its preparation, in a manner also generallyknown, with a graft shell whose composition corresponds to that of thebase polymer or which is compatible with the base polymer. This furtherimproves the processing properties and performance characteristics ofthe toughened blend.

As a rule, the elastomeric graft polymers are prepared by emulsionpolymerization. This process gives very finely divided polymers having amean particle diameter of from 0.1 to less than 1 μm.

However, this material gives only molding materials having high surfacegloss. Frequently, however, dull moldings are desirable, so that dullingagents, such as silica gel or chalk, must be concomitantly used,although they have an adverse effect on the mechanical properties of themolding materials and of the moldings.

EP-A 269 324 discloses graft polymers having a relatively large particlediameter and comprising an elastomeric core polymer and an outer graftshell of a polymer which is compatible with the abovementionednonresilient base material. Although molding materials containing theserefracting particles have a dull surface, the process for thepreparation of the particles is extremely inconvenient. In this process,the polymer particles of the grafting core which have initially formedin the emulsion polymerization are swollen with freshly added monomer,after which the polymerization is continued. This process can berepeated until the desired particle size is reached. Finally, theseparticles are provided with the graft shell which makes them compatiblefor mixing with the matrix polymer.

Furthermore, the preparation of the elastomeric particles by theemulsion polymerization process has in general the disadvantage thatsome of the emulsifiers required for this purpose remain in the polymerand are exuded from the moldings produced therefrom, with formation ofundesirable coatings.

It is an object of the present invention to remedy the deficienciesdescribed. It is a particular object of the present invention to provideelastomeric particulate graft polymers which make it possible toincrease the impact strength of nonresilient polymers and at the sametime perform the function of dulling agents.

We have found that these objects are achieved by the elastomericparticulate graft polymers defined at the outset.

We have also found a process for the preparation of these graftpolymers, their use as components for blends with macromolecular basematerials, such blends themselves and moldings comprising such blends.

The novel graft polymers are preferably obtained as follows: the liquidmonomer or liquid monomer mixture M_(K) which is to be polymerized togive the core polymer P_(K) is mixed with water and a protectivecolloid. The polymerization initiator is added either now or only afterdispersing of the monomer or after heating of the dispersion. Adispersion of very small monomer droplets in water is prepared from theheterogeneous mixture by thorough stirring at high speed, intensivemixers of any design being suitable for this purpose. The desiredparticle size within the defined range can be determined, for example,by preparing optical micrographs and counting the number of particleswhich have a certain diameter.

The polymerization is initiated by heating the dispersion. The reaction,which is now carried out with moderate stirring during which thedroplets are no longer further dispersed, is continued until theconversion is more than 50%, preferably more than 85%, based on M_(K).

Once the polymerization of the grafting core is complete, the reactionis continued in a manner known per se with the monomers M_(s) from whichthe corresponding shells S, consisting of the polymer P_(s), are formed.However, the grafting may also be started as early as when thepolymerization conversion of the monomer M_(K) is still incomplete andis more than 50%, preferably more than 85%. In this case, the shell andcore form a more fluid transition compared with the sharper boundary ofthe core-and-shell polymer in the case of the initially completeconversion of the core monomers.

If the graft polymer has only a single shell, which is generallysufficient, this consists of the material P_(a). In some cases, chieflywhen the grafting cores are relatively small and when it is desired tointroduce a larger amount of the core polymer P_(K) into the particles,multishell graft polymers having the structure P_(K) -P_(a) -P_(K)-P_(a) are preferred, and the inner shells may also consist of otherpolymers P_(x) in order thus to modify and hence improve the propertiesof the graft polymers.

Dispersing of the monomers M_(K) is carried out as a rule at from 0° to100° C., preferably at room temperature, and as a rule from 0.5 to 10 kgof water are used per kilogram of the monomers.

The protective colloids suitable for stabilizing the dispersion arewater-soluble polymers which coat the monomer droplets and the polymerparticles formed therefrom and thus protect them from coagulation.

Suitable protective colloids are cellulose derivatives, such ascarboxymethylcellulose and hydroxymethylcellulose,poly-N-vinylpyrrolidone, polyvinyl alcohol and polyethylene oxide,anionic polymers such as polyacrylic acid, and cationic ones, such aspoly-N-vinylimidazole. The amount of these protective colloids ispreferably from 0.1 to 5% by weight, based on the total weight of themonomers M_(K). Low molecular weight surface-active compounds, forexample those of the type comprising the anionic and cationic soaps, areas a rule unsuitable for the preparation of the novel graft polymerssince they lead to polymer particles having smaller diameters, asobtained in the emulsion polymerization.

Suitable polymerization initiators are free radical formers, inparticular those which have marked solubility in the monomers and whichpreferably have a half-life of 10 hours when the temperature is from 25°to 150° C. (ten-hour half-life at from 25° to 150° C.). For example,peroxides, such as lauroyl peroxide, peroxosulfates, tert-butylperpivalate and azo compounds, such as azobisbutyronitrile, aresuitable. It is possible to use different initiators for the preparationof the grafting core and of the graft shells. The amount of theinitiators is in general from 0.1 to 2.5% by weight, based on the amountof the monomers.

The reaction mixture furthermore preferably contains buffer substances,such as Na₂ HPO₄ /NaH₂ PO₄ or sodium citrate/citric acid, in order toestablish a pH which remains essentially constant. As a rule, molecularweight regulators, such as ethylhexyl thioglycolate or tert-dodecylmercaptan, are also added during the polymerization, in particular ofthe shell-forming monomers Ms.

The temperature for the polymerization of the monomers M_(K) to give thecore consisting of P_(K) is as a rule from 25° to 150° C., preferablyfrom 50° to 120° C. Grafting of the shells onto the core is generallycarried out at from 25 ° to 150° C., preferably from 50° to 120° C. Thelower limits of these ranges correspond to the decompositiontemperatures of the polymerization initiators used in each case.

Suitable elastomeric polymers P_(K) of the grafting core material arechiefly those which are composed of from 50 to 100% by weight, of C₂-C₃₆ -alkyl acrylates, preferably n-butyl acrylate and/or 2-ethylhexylacrylate. In addition to these soft monomers, hard monomers, such asmethyl acrylate, the C₁ -C₁₂ -alkyl methacrylates, styrene andα-methylstyrene, acrylonitrile and methacrylonitrile, are also suitable,in amounts of up to 50% by weight.

Such polymers are obtained with from about 0.1 to 10% by weight ofbifunctional or polyfunctional comonomers, eg. butadiene and isoprene,divinyl esters of dicarboxylic acids, such as those of succinic acid andadipic acid, diallyl and divinyl ethers of bifunctional alcohols, suchas those of ethylene glycol and of butane-1,4-diol, diesters of acrylicacid and methacrylic acid with the stated bifunctional alcohols,1,4-divinylbenzene and triallyl cyanurate. The acrylates oftricyclodecenyl alcohol (dihydrodicyclopentadienyl acrylate) of theformula I ##STR1## and the allyl esters of acrylic acid and ofmethacrylic acid are particularly preferred.

In the novel graft polymers, the weight ratio of the sum of all shellsto the core is from about 0.05:1 to 2.5:1. The mean diameter of theparticles is from about 1 to about 150 μm, preferably from 2 to 50 μm,particularly preferably from 5 to 30 μm. The mean diameter correspondsto the D₅₀ value, where 50% by weight of all particles have a smallerdiameter, and 50% by weight a larger diameter, than the diameter whichcorresponds to the D₅₀ value. In order to characterize the particle sizedistribution, in particular its width, the D₁₀ value and the D₉₀ valueare often stated in addition to the D₅₀ value. 10% by weight of allparticles are smaller, and 90% by weight larger, than the D₁₀ diameter.Similarly, 90% by weight of all particles have a smaller diameter, and10% by weight a larger diameter, than that which corresponds to the D₉₀value.

The novel particulate graft polymers are used mainly as additives forbrittle thermoplastic macromolecular base materials B. On the one hand,their impact strength is improved as a result; on the other hand,molding materials having reduced surface gloss and accordingly dullmoldings are obtained owing to diffuse reflection (scattering) of thelight by the large particles.

The elastomeric particles are incorporated into the melt of B so thatthe resulting molding material is composed of the thermoplastic matrix Band the graft polymer particles dispersed therein. In order to make theelastomeric core polymers P_(K) compatible with the base polymer for thestated purposes, as a rule their outer graft shell Sa consists of thesame material as the base polymer or a very similar material to saidpolymer. The technically most important base polymers are homopolymersof styrene, of methyl acrylate, of C₁ -C₄ -alkyl methacrylates and ofacrylonitrile, copolymers of these monomers and further comonomers, suchas methacrylonitrile, ie. these monomers and monomer mixtures aresuitable for the synthesis of the outer graft shell S_(a), depending onthe composition of the base polymer B.

If the outer shell is to be, for example, relatively hard, intermediateshells of a softer material may be preferable. Furthermore, a shellcomprising a soft material, for example the core material, may followthe first hard graft shell, thus frequently making it possible furtherto improve the properties of the thermoplastic molding materialsprepared from B and the graft polymer particles and of the moldingsproduced therefrom. The relationships between the nature of the twocomponents in the molding materials and the material propertiesfurthermore correspond to those known for the base material and graftpolymers which are prepared by emulsion polymerization.

This also applies to base materials other than the stated ones, forexample polyesters, polyamides, polyvinyl chloride, polycarbonates andpolyoxymethylene. In these cases, compatible and partly compatible graftshells S_(a) can readily be determined by a few preliminary experiments.

Compatibility is understood as meaning miscibility at the molecularlevel. A polymer is considered to be compatible with another one if, inthe solid state, the molecules of the two polymers are randomlydistributed, ie. if the concentration of one polymer along any vectorneither increases nor decreases. Conversely, a polymer is considered tobe incompatible if, in the solid state, two phases which are separatedfrom one another by a sharp phase boundary are formed. Along a vectorintersecting the phase boundary, the concentration of one polymerincreases abruptly from zero to 100% and that of the other polymerdecreases from 100% to zero.

Between the two extremes, there are fluid transitions. Although theyhave a phase boundary, it is ill defined. Mutual partial penetration ofthe two phases occurs at the boundary. Accordingly, the concentration ofone polymer increases more or less rapidly from zero to 100% along avector intersecting the phase boundary and that of the other polymerdecreases more or less rapidly from 100% to zero.

Partial compatibility, as frequently encountered in technicallyimportant polymers, is the term also used to describe this latter case.

Examples of partly compatible polymers are the pairs polymethylmethacrylate/copolymer of styrene and acrylonitrile, polymethylmethacrylate/polyvinyl chloride and polyvinyl chloride/copolymer ofstyrene and acrylonitrile and the three-phase systempolycarbonate/polybutadiene/copolymer of styrene and acrylonitrile(=polycarbonate/ABS).

Further information on the definition of compatibility of polymers andin particular the solubility parameter as a quantitative measure isgiven, for example, in Polymer Handbook, Editors J. Brandrup and E. H.Immergut, 3rd edition, Wiley, New York 1989, pages VII/519-VII/550.

The novel graft polymers are used as a rule in amounts of from 1 to 60,preferably from 2 to 45, % by weight, based on the amount of their blendwith the base polymer, for toughening. Moldings comprising such blendsare highly light-scattering and therefore particularly dull to opaque.

If a dulling effect is desired in combination with high transparency,concentrations of from 2 to 10% by weight of the graft polymers arepreferred. Since these low concentrations result in only a relativelysmall increase in the impact strength, conventional, very finely dividedelastomeric modifiers may be concomitantly used in this case, in theconventional amounts minus the amount of novel graft polymer used as adulling agent.

Opaque polymers which already contain impact modifiers, for examplepolybutadiene-modified styrene/acrylonitrile copolymer (=ABS), polyalkylacrylate-modified styrene/acrylonitrile copolymer (=ASA) orstyrene/acrylonitrile copolymer (=AES) modified withethylene/propylene/diene monomer (EPDM) can also be dulled by theconcomitant use of the novel graft polymers.

The novel particles achieve a dulling effect without having a markedadverse effect on mechanical properties, as is to be observed in thecase of conventional dulling agents, such as chalk or silica gel.

Owing to their higher molecular weight and greater bulk of themolecules, the protective colloids used in the preparation of the corepolymers have far less tendency than the low molecular weightemulsifiers to migrate to the surface of the plastic. High molecularweight protective colloids therefore have far less tendency to exudefrom a molding.

In addition, the molding materials modified with the novel particles andthe moldings produced therefrom have the advantages of improvedprintability and antiblocking properties, ie. those surfaces of themoldings which are roughened by the particles do not adhere to oneanother. This effect due to adhesion is known, for example, for plasticsfilms. Films containing novel particles and stacked one on top of theother can be readily separated from one another, in contrast to filmswhich do not contain such particles.

The molding materials may furthermore contain additives of all types.Examples are lubricants and mold release agents, pigments, flameproofingagents, dyes, stabilizers, fibrous and pulverulent fillers andreinforcing agents and antistatic agents, each of which are added in theconventional amounts.

The novel molding materials can be prepared by mixing methods known perse, for example by incorporating the particulate graft polymer into thebase material at above the melting point of the base material, inparticular at from 150° to 350° C., in a conventional mixing apparatus.Moldings having reduced surface gloss (dullness) and high impactstrength can be produced from the novel molding materials. No separationof the polymer components occurs in the molding.

EXAMPLES Example 1

Particulate graft polymer of n-butyl acrylate (core) andstyrene/acrylonitrile (shell); according to the invention, polymer A

    ______________________________________    1230  g     of water,    8.6   g     Na.sub.2 HPO.sub.4.12 H.sub.2 O                                 as a buffer system    3.2   g     NaH.sub.2 PO.sub.4.12 H.sub.2 O    1.6   g     of dilauroyl peroxide as an initiator    100   g     of polyvinyl alcohol solution (10% by weight in                water, degree of hydrolysis 88%, average molecular                weight about 127,000) as protective colloid    600   g     of n-butyl acrylate as a core monomer and    9.0   g     of dihydrodicyclopentadienylacrylate as a cross-                linking agent    ______________________________________

were combined in the stated order under nitrogen and stirred for 40minutes with a high-speed stirrer (dissolver stirrer, 3500 rpm, 5 cmtoothed disk). At the same time, the mixture was gradually heated to 73°C. This resulted in the formation of monomer droplets having a meandiameter of 10 μm, as determined microscopically on a sample.

This dispersion was transferred to another kettle and heated there to87° C. in the course of 2 hours with moderate stirring, the core polymer(conversion about 95%) being formed.

The mixture was then mixed with a mixture of

    ______________________________________    280   g     of styrene                                 as shell monomers    120   g     of acrylohitrile    0.5   g     of tert-butyl perpivalate as initiator and    0.5   g     of 2-ethylhexyl thioglycolate as a molecular weight                regulator    ______________________________________

at 70° C. in the course of 10 minutes with moderate stirring, and theresulting mixture was then kept at 70° C. for 2 hours and then heated to85° C. in the course of 2 hours.

The particles of the graft polymer thus obtained (60% by weight ofn-butyl acrylate, 28% by weight of styrene and 12% by weight ofacrylonitrile, crosslinked) had a mean particle diameter D₅₀ of 10 μm, adiameter D₁₀ of 4 μm and a diameter D₉₀ of 25 μm.

The suspension obtained was incorporated as such into the polymer, asdescribed below.

Example 2

Moldings A base material B was thoroughly mixed in the usual manner inan extruder (ZSK 30, Werner and Pfleiderer) at 260° C. with the polymersE1 and E2 prepared by conventional emulsion polymerization, and themixture was melted. The polymer A was introduced into this polymer meltcontinuously in the form of its emulsion, and the water content of theemulsion was removed by means of dewatering apparatuses along theextruder, after which the melt was extruded. The product obtained aftercooling and granulation was injection molded on an injection moldingmachine at a melt temperature of 220° C. and a mold temperature of 60°C. to give circular disks having a diameter of 6 cm and a thickness of0.2 cm of standard small bars (cf. DIN 53453), the properties of whichwere tested.

The polymers were the following materials (% by weight in each case):

Polymer A: elastomeric graft polymers according to Example 1

Polymer B: hard thermoplastic copolymer comprising 65% of styrene +35%of acrylonitrile, viscosity number according to DIN 53 726: 80 ml/g(0.5% strength in dimethylformamide at 23° C.).

Emulsion polymer E1: elastomeric graft polymer prepared in aconventional manner by emulsion polymerization. Comprising 60% ofn-butyl acrylate, crosslinked with tricyclodecenyl acrylate (core)/30%of styrene +10% of acrylonitrile (shell), mean particle diameter D₅₀about 0.5 μm

Emulsion polymer E2: elastomeric graft polymer prepared in aconventional manner by emulsion polymerization. Comprising 60% ofbutadiene (core)/28% of styrene +12% of acrylonitrile (shell), meanparticle diameter D₅₀ about 0.35 μm.

The ratios in the polymer blends and the properties of the moldingsproduced from them are shown in the table below.

    ______________________________________             Compostion of the                             Notched  Light    Experi-  polymer blend   impact   reflec-    ment      % by weight!   strength.sup.1)                                      tion.sup.2)    No.      B     A       E1  E2     kJ/m.sup.2 !                                             %!    ______________________________________    1*       50    --      50  --    20.0   87    2*       50    --      --  50    18.0   85    3        50    5       45  --    19.6   12    4        45    5       --  45    18.5   10    ______________________________________     .sup.1) measured according to DIN 53 453 at 23° C. on standard     small bars with milled notch     .sup.2) measured according to DIN 67530 with a Gonio GPZ photometer from     Carl Zeiss, at an angle of 45° C. (reflection)     *for comparison

It can be seen that those molding materials which contain exclusivelyconventional graft polymer as the impact modifier have high lightreflection (Experiment No. 1* and 2*). In contrast, molding materials inwhich 10% of the conventional graft polymer E 1 or E 2 is replaced bythe novel graft polymer particles A having a large diameter exhibitconsiderably lower reflection in combination with a similar notchedimpact strength (Experiments No. 3 and 4).

We claim:
 1. A particulate elastomeric graft polymer which increases theimpact strength of and at the same time performs the function of adulling agent for macromolecular base materials B comprising B-1)polymers containing polymerized sytrene, methyl acrylate, C₁ -C₄ -alkylmethacrylate, acrylonitrile or methacrylonitrile or mixtures thereof,orB-2) impact-resistant styrene copolymers of ASA, ABS or AES ormixtures thereof, or B-3) polyesters, polyamides, polyvinyl chloride,polycarbonates or polyoxymethylene or mixtures thereof,or mixturesthereof and consists of a core K and one or more shells S, in whichtheparticles of the particulate elastomeric graft copolymer have a meanparticle diameter of from 1 to 150 μm, the core K consists of acrosslinked elastomeric polymer P_(K), the outer shell S_(a) is formedby a polymer P_(a) which is compatible or partly compatible with thebase material B and any further shells present may be composed of P_(K),P_(a) or further polymers P_(x),obtained by a) dispersing the liquidmonomer or liquid monomer mixture M_(K) corresponding to the polymerP_(K) in water using a protective colloid to give a dispersion ofdroplets having a mean diameter of from 1 to 100 μm, b) polymerizing thedroplets by means of a polymerization initiator to a conversion of morethan 50%, based on the amount of M_(K), and c) subjecting the mixtureobtained in stage b) to graft polymerization with sequential addition ofthe monomers or monomer mixtures M_(S) corresponding to the graft shellsS until the outer shell S_(a) has been produced using the correspondingmonomers M_(a).
 2. A graft polymer as claimed in claim 1, in which thecore polymer P_(K) is composed offrom 50 to 100% by weight of a C₂ -C₁₂-alkyl acrylate, from 0 to 10% by weight of a crosslinking monomer, andfrom 0 to 50% by weight of one or more further monomers, based in eachcase on the amount of P_(K).
 3. A graft polymer as claimed in claim 1,in which the polymer P_(a) of the outer shell contains polymerizedstyrene, methyl acrylate, C₁ -C₄ -alkyl methacrylate, acrylonitrile ormethacrylonitrile or mixtures thereof.
 4. A graft polymer as claimed inclaim 1, in which the weight ratio of the sum of all shells to the coreis from 0.05:1 to 2.5:1.
 5. A process for the preparation of aparticulate graft polymer which is suitable for improving the quality ofmacromolecular base materials B, by polymerization of a core Kcomprising monomers M_(K) which form elastomeric polymers P_(K) andgrafting with one or more shells S, the outer shell S_(a) being composedof a polymer P_(a) which is formed from the monomers M_(a) and iscompatible or partly compatible with the base material B, bypolymerization in the aqueous phase in the presence of a free radicalpolymerization initiator, whereinthe monomer or a monomer mixture M_(K)is very finely dispersed in water using a protective colloid, themonomers M_(K) are polymerized to a conversion of more than 50% and thismixture containing the grafting cores K is subjected to graftpolymerization by sequential addition of the monomers or monomermixtures M_(s) corresponding to the graft shells S until the outer shellS_(a) has been produced using the corresponding monomers M_(a) wherebythe mean particle size of the particulate graft polymer produced is 1 to150 μm.
 6. A molding material comprising a macromolecular base materialB containing a graft polymer as claimed in claim
 1. 7. A moldingcomprising a molding material as claimed in claim
 6. 8. A method forimproving the mechanical properties of macromolecular base materials B,wherein the graft polymers of claim 1 are incorporated into the basematerials B.
 9. A method for matting the surface of macromolecular basematerials B, wherein the graft polymers of claim 1 are incorporated intothe base material B.
 10. The particulate graft polymer of claim 1wherein the particles of the particulate elastomeric graft copolymerhave a particle size of from 2 to 50 μm.
 11. The particulate graftpolymer of claim 10 wherein the particles of the particulate elastomericgraft copolymer have a particle size of from 5 to 30 μm.
 12. The processfor the preparation of a particular graft polymer of claim 5 wherein themean particle size of the particulate graft polymer produced is 2 to 50μm.
 13. The process for the preparation of a particular graft polymer ofclaim 12 wherein the mean particle size of the particulate graft polymerproduced is 5 to 30 μm.